1. Field of the Invention
This invention relates to photolithography of substrates, and more particularly, the invention relates to photolithography with phase conjugate optics for imaging onto a substrate.
2. Description of the Prior Art
With the evolution of microelectronic devices continuing, the desire to form individual structures of smaller and smaller dimensions with more and more complexity has created a demand for extremely fine line lithography. Projection photolithography is currently replacing proximity printing as a means for reproducing images with the finest achievable resolution. With the step and repeat type systems currently available, the optics utilized within the projection photolithography machines has approached the ultimate performance level for apparatus of this type. The optical systems involved in photolithography with projection printing consists of refractive and reflective elements. These systems do not reach diffraction limited performance. The semiconductor manufacturing industry requirements are for a production capability with line widths within the submicron range. Electron beam and x-ray lithography techniques are being developed in order to meet the demand for submicron structures.
A photolithographic system utilizing a step and repeat technique is shown in U.S. Pat. No. 3,572,925, issued to Ables on Mar. 30, 1971, and entitled "Step and Repeat Camera with Computer Controlled Film Table". Although Ables shows photolithographic printing and the reproduction of images on a glass substrate, a semiconductor wafer can be substituted therefor. Further, the Ables patent shows a plurality of optical paths, although a single optical path is generally utilized by the machines currently available. The image which is present on a reticle or mask is reproduced onto the glass substrate with a electromagnetic radiation sensitive layer thereon. The photosensitive layer has portions thereof exposed by light which passes first through the reticle and finally impinges onto the electromagnetic radiation sensitive layer on the photographic plate. The X-Y table which supports the photographic plate is then repositioned and the process of exposing another image is continued. The positioning of the X-Y table is accomplished by the utilization of laser interferometers along with a computer to provide positional information for controlling motors which are connected and controlled by the computer. The computer maintains a positional record of the X-Y table by counting fringe lines which are detected by the laser interferometer. The currently available direct step-on-wafer machines are capable of reproducing the image taken from a reticle or mask and reproducing it onto the wafer through an optical path. However, as discussed above, these systems are limited by the optics utilized and are capable of reproducing geometeries regularly which have the dimension of greater than a few microns.
A phenomenon resulting from nonlinear optical mixing is called "phase conjugation". It is a phenomenon which results from the use of nonlinear techniques for real time processing of electromagnetic fields. Phase conjugation is a term which is utilized to describe a phenomenon involving reversal of direction of propagation and in addition a phase reversal of an incident beam of electromagnetic radiation. In one form of phase conjugation, known as degenerate four wave mixing, the incident incoming beam is mixed with a pair of pump beams, as will be discussed herein, and a fourth output beam is generated within the nonlinear medium which is the phase conjugate of the incoming or incident beam. The properties of the conjugated beam are such that as the wave propagates the phase of the conjugated beam undergoes a time reversal with respect to the phase properties of the incoming or incident beam. It has been further noted that the incoming beam can be amplified by the pump beams within the linear medium to produce a conjugated beam which has a greater intensity or amplitude than the incoming or incident beam. As the conjugated beam propagates away from the nonlinear medium, its phase condition is substantially identical with the phase condition of the incoming beam at the same distance from the nonlinear medium along their respective optical paths. In other words, if the incoming beam of electromagnetic radiation has a particular phase condition at time T1 and the waves in the beam require a time interval .DELTA.T to reach the nonlinear medium at time T2, the conjugate beam has the identical particular phase condition of the incoming beam at T1 as it progagates away from the nonlinear medium along its optical path at a time equal to the time of exit of wave of electromagnetic radiation from the nonlinear medium at time T3 plus the time interval .DELTA.T. This phenomenon has been described as time reversal in that the phase component of the electromagnetic radiation of the conjugate beam has experienced a reversal of its time variable. The phenomena associated with phase conjugation are described in more detail in the papers by Yariv and Pepper, published in Optics Letters Vol. 1, No. 1, in July 1977, and entitled "Amplified Reflection, Phase Conjugation, and Isolation in Degenerative Four Wave Mixing", and Yariv published in the IEEE Journal of Quantum Electronics, Vol. QE-14, No. 9, on September 1978 and entitled "Phase Conjugate Optics and Real Time Holography, which are incorporated by reference hereinto.
The phase conjugation phenomenon can also be utilized within a photolithographic system to reduce the speckle. Speckle is a term used to describe a phenomenon in which the scattering of light from a highly coherent source, such as a laser, by a rough surface, or in inhogenous medium generates a random intensity distribution of light that gives the surface or medium a granular appearance.
In photolithography, even the mild speckle which results from the incidence of the electromagnetic radiation wth the mask irradiated can cause defects in the reproductions of the images present on the mask.